As shown in FIG. 4, in a conventional magnetic disk device 1, a spindle motor (not shown) for holding a magnetic disk 2 between the spindle motor and a disk clamp 3 and rotating the magnetic disk 2, a carriage 4 having a magnetic head 4a for recording and reproducing information on the magnetic disk 2, and an actuator 5 for moving the magnetic head 4a via the carriage 4 are placed on a chassis 6.
An arm 8 having a suspension 7 for supporting the magnetic head 4a is mounted on one end of the carriage 4, the carriage 4 is pivotally supported by a shaft 9 rotatably on the chassis 6, and a coil 10 is mounted on the other end of the carriage 4, the shaft 9 being placed between the ends.
The actuator 5 is a voice coil motor (VCM) and is constituted by the coil 10 placed on the other end of the carriage 4, and a magnetic circuit which applies a magnetic field on the coil 10 and generates driving force for rotating the carriage 4 around the shaft core of the shaft 9. The magnetic circuit is formed by a pair of yokes 11 and a magnet 12, in which the yokes 11 are opposed so as to form a prescribed gap in which the other end of the carriage 4 can move, and the magnet 12 is fixed on one of the opposing surfaces of the yokes 11. Reference numeral 11a denotes shafts for forming the gap between the yokes 11, and reference numeral 11b denotes stoppers which are placed on the outer circumference of the shafts 11a to suppress excessive rotation of the carriage 4. The shafts 11a and the stoppers 11b are provided in pairs.
A ramp having an up grade and a down grade along the circumferential direction is provided on the circumference where, the tip of the suspension 7 is moved on the side of the magnetic disk 2. When the magnetic disk 1 is not operated, a load tab 7a at the tip of the suspension 7 is moved to a retreat position 13a after moving over the top of the ramp 13 to permit the suspension 7 (thus, the magnetic head 4a) to retreat from the magnetic disk 2.
When the suspension 7 is caused to retreat from the magnetic disk 2, the worst condition of unloading is that power supply to the magnetic disk device 1 is abruptly interrupted by an accident. In this case, counterelectromotive force of the spindle motor which is continuously rotated by counterelectromotive force is supplied to the coil 10, the carriage 4 is rotated by obtaining driving force of the coil 10, and the load tab 7a of the suspension 7 is caused to reach the retreat-position 13a. 
However, when the magnetic disk device 1 is small, since the magnetic disk 2 has a small diameter and a light weight, inertial force is small and the rotation of the spindle motor is made for a short time. Thus, sufficient counterelectromotive force cannot be obtained and assist force is necessary to allow the load tab 7a of the suspension 7 to climb the ramp 13.
Hence, as shown in FIGS. 5 and 6, on a corner of the carriage 4 on the side of the magnetic disk 2, a lock part 14 is provided which is made of a magnetic material and is shaped like a letter L, and magnets 15 and 16 covered with rubber or resin are placed near the stopper 11b so as to face the sides of the lock part 14 shaped like the above described letter L. Thereby, when power supply is abruptly turned off, attractive force of the magnet 15 for attracting the lock part 14 is added to driving force generated on the coil 10, and the load tab 7a securely moves over the ramp 13. When the magnetic disk device 1 is not operated (when the magnetic head 4a retreats), the magnet 16 attracts the lock part 14 to lock the carriage 4 such that external impact does not move the suspension 7 (thus, the magnetic head 4a) onto the magnetic disk 2.
However, in recent years, high performance and miniaturization of magnetic disk devices have increased in demand and it has been more difficult to obtain a space for placing the above-described magnets 15 and 16. Although it is considered that the magnets 15 and 16 are reduced in size to obtain a space for placement, the small magnets 15 and 16 are hard to handle.
For this reason, the subject has been development of a magnetic disk device comprising a locking mechanism for miniaturization that permits an arm with a magnetic head to be placed at a retreat position on the side of a disk during unloading and non-operation.